1. Field of the Invention
The present invention relates to the field of integrated circuit manufacturing, and more specifically, to interconnection metals for integrated circuits.
2. Description of Relevant Art
Modern integrated circuits are made up of literally millions of active devices such as transistors and capacitors formed in a semiconductor substrate. These devices are initially isolated from one another but are later interconnected together to form functional circuits. The quality of the interconnection metal drastically effects the performance and reliability of the fabricated integrated circuit. Interconnection metals are increasingly determining the limits in performance, density and reliability of modern ultra large scale (ULSI) circuits.
FIG. 1 is a cross-sectional illustration of an interconnect structure which is presently widely used in the semiconductor industry. It comprises a silicon substrate 101 in which active devices (not shown) are formed. An interlayer dielectric (ILD) 103 is formed over the substrate to isolate aluminum interconnection lines 105 and 107 from active devices formed below. The aluminum interconnection lines interconnect various devices to form functional circuits. The aluminum interconnection lines are typically coupled to the substrate by metal plugs 109 and 111.
Aluminum and its alloys have been widely used as interconnection lines 105 and 107 in interconnect structures because they have good resistivity (.about.3.0.mu..OMEGA.-cm) and they have good adhesion to SiO.sub.2, which is typically used as an ILD. Additionally, aluminum doped with a small amount of copper does not diffuse through ILD 103 and interact with the substrate below. Unfortunately, aluminum offers poor resistance to electromigration which increases the potential for open circuits from voids or short circuits from hillocks. Additionally, aluminum thin films suffer from stress migration which can cause voids and hillocks at relatively low temperatures. Hillocks can cause interlevel and intralevel shorts in multilevel integrated circuits. Still another problem with aluminum alloy lines is that they are susceptible to humidity-induced corrosion.
In an attempt to improve the performance, reliability, and density of interconnections, alternative interconnection metals to aluminum and aluminum alloys have been proposed. Pure copper has been proposed as a substitute for aluminum metalization. Pure copper has an extremely low resistivity (.about.1.7.mu..OMEGA.-cm). A low resistivity interconnection metal improves performance of an integrated circuit by increasing its speed. Additionally, pure copper is resistant to electromigration which makes it a much more reliable interconnection metal than aluminum and aluminum alloys.
Unfortunately, pure copper interconnections have several shortcomings which make them ill-suited for use in high performance reliable integrated circuits. First, pure copper readily oxidizes whenever oxygen is present. Oxidation of copper interconnections increases the electrical resistivity of the interconnection, thereby decreasing the performance of the fabricated circuit. It is to be appreciated that interconnections can be exposed to oxygen during a number of steps in the integrated circuit manufacturing process For example, interconnections are exposed to oxygen in air whenever wafers sit idle between process modules. Interconnections can also be exposed to oxygen during the formation of oxide based (SiO.sub.2) interlayer dielectrics. Oxidation of copper is especially troublesome because the reaction between copper and oxygen is not self limiting, unlike other metals such as aluminum, and therefore the entire interconnection can become oxidized which drastically increases the resistance of the interconnection. Another problem with pure copper interconnections is that they easily corrode (in addition to oxidizing, a form of corrosion) and cause reliability problems. Still another problem with copper interconnections is that they readily diffuse through SiO.sub.2 and other ILD materials such as polyimides. Pure copper interconnections, therefore, require a barrier layer to prevent diffusion. Barrier layers add expense and process complexity to the fabrication of an integrated circuit.
In a similar manner, pure silver has been proposed as a substitute for aluminum alloy interconnections. Silver has an extremely low resistivity (.about.1.6.mu..OMEGA.-cm). Unfortunately, however, like pure copper, pure silver readily oxidizes and corrodes creating reliability problems.
Thus, what is desired is an interconnection metal which has a low resistivity but at the same time is resistant to void and hillock formation, oxidation, and corrosion.